Prostitution is sex in exchange for money. One party is willing to pay money to have sex. The other party is willing to receive money for sex. If both parties adults, government should not take away their freedom to engage in the exchange. Of course, child prostitution and forced prostitution should remain illegal.

legalization makes prostitution safer

If sex workers are not treated as criminals, they can seek the protection of law enforcement without fear of being arrested. Sex workers should be registered. This way, it will be easier to for law enforcement and potential clients when sex workers are minors, when they are forced or when they are illegal immigrants.

Con, you can still disagree. Your debate doesn't necessarily mean you're for or against it immediately. By debating against it you could find out something about prostitution you didn't know and change your mind about legalizing it or find something that will further your stance on legalizing it. It's better for a man's mind to change his opinions and think of how the one he disagrees with thinks.